51       mi  B   M   MOD   ME2 

5lo'i  mmrmrTmrmsioiooY 

^'^^   OF  THE  EYE,  ^ITH  HINTS  FOR 
1 1  THE  PRESERVATION  OF  THE 


mm   EYE-SIGHT 


'Q^ 


Urn 


f  crO 


THE  LIBRARY 

OF 

THE  UNIVERSITY 

OF  CALIFORNIA 


GIVEN  WITH  LOVE  TO  THE 

OPTOMETRY  LIBRARY 

BY 

MONROE  I.  HIRSCH,  O.D.,  Ph.D. 


Anatomy  and  Physiology 
OF  THE  Eye 


WITH   HINTS   FOR   THE 


PRESERVATION  OF  THE  EYESIGHT 


THE    KKLAITVl';    i'OSmoNS    UF     IHI-;    KVKBAl.LS,    MUSCLIiS 
AND    NKRVKS 


J,    KKEDERICK     HERBERT,    Pvl.   D. 


Third  Edition 


Published  at  1516  Locust  Street 
philadelphia,  pa. 


COPVKIGHl     K)Or 


J.   FRIiDKKlCK   HHRHEKT 


^e- 


u  .■ 


PREFACE, 


The  major  portion  of  this  monograph  was  delivered  as 
a  lecture  to  the  young  men  of  the  Cheltenham  Military 
Academy,  at  Ogontz,  Pa.  It  was  illustrated  with  espe- 
cially prepared  lantern  slides.  Some  of  the  illustrations 
were  borrowed  from  scientific  treatises. 

Most  books  on  the  eye  arc  so  technical  that  they  are 
practically  sealed  to  all  but  the  specialist,  who  is  com- 
pelled to  have  a  knowledge  of  anatomy,  physiology,  and 
optics. 

In  this  work  it  has  been  the  author's  endeavor  to  omit 
all  strictly  scientific  and  medical  conclusions. 

In  an  effort  to  popularize  facts  and  accepted  theories, 
he  has  made,  use  of  his  own  library  on  the  subject,  as  well 
as  gathered  all  available  material.  In  con.sequence,  he 
does  not  claim  entire  originalit}'  in  the  work. 

He  trusts  that  his  attempt  to  render  this  branch  of 
science  intelligible  to  the  general  reader  will  meet  with 
success. 

This  done  he  will  feel  repaid  for  all  the  labor  involved 
in  the  preparation  of  a  work  of  this  kind. 


TABLE  OF  CONTENTS. 


PACK 

Prkfack,                , 3 

Tahm-;  ok  Contknts,      5 

List  ok  Ii.i.ustratioxs, 6 

Introduction,          .    .           7-S 

The  Human  Evk.     Part  I,   .    .    .        9-16 

Thk  Human  Evk.     Part  H,     17-22 

Nkrvks  of  the  Eye 23 

Arteries  of  the  P^ve, 23 

Veins  of  the  Eve 24 

PhysioloCxV  of  Vision, 24 

Visual  Angle, 25 

Visual  AcutenEvSS, 25-26 

Accommodation, 26-28 

The  Emmetropic,  or  Normal  Eve 29 

HVPERMETROPIC,    OR   FaR-SiGHTED   EvE,                   ...          .  30-31 

Mvopic.  OR  Near-Sighted  Eve,         32-33 

Astigmatism,            34-40 

Presbyopia,  or  Old  Sight, 41 

Hygiene  of  the  Eve,      42-46 

Light,                                 47  4S 

Refraction  of  Light,         48-49 

Reflection  of  Light, 49 

Spectacle  Lenses, 50-53 

Optical  GlAvSS,               54-55 

vSpectacle  Lens  Grinding, 56-58 

History  of  Spectacles, 59-68 


ILLUSTRATIONS. 


PAGE 

Frontispiece,   showing   relative   positions  of    the    Eyeballs, 

Muscles,  and  Nerves. 

Fig.    I.  The  Human  Eye  (Part  I), 9 

2,  The  Photographic  Camera jo 

"      3.  Front  View  of  Rye,    •       ■       11 

4.  Verticai.  Section  of  Eyelid, 12 

5.  Posterior  Portion  of  Eyelids 13 

6.  Lacrymal  Apparatus,      14 

7.  Eyeball  in  Position  in  Orbital  Cavity,  ....  15 
"      S.  The  Human  Eye  (Part  II), 17 

9.  Retina, 19 

"    10.  Section  through  the  Retina, 20 

"    II.  Base  of  the  Human  Brain,      23 

"    12.  Visual  Acuteness 25 

"    13.  Accommodation, 26 

"     14.  Accommodation 27 

"     15.  Emmetropic,  or  Normal  P^ye 29 

"     16.  Hypermetropic,  or  Far-Sighted  I^^vk 30 

"     17.  Hypermetropic,  or  Far-Sighted  P^ve,   ...  31 

"    iS.  Myopic,  or  Near-Sighted  Eye, 32 

"    19.  Myopic,  or  Near-Sighted  Eye, 33 

"    20.  Test-Card  for  Astigmatism, 34 

"    21.  Test-Card  for  Astigmatism, 35 

"    22.  Cylindrical  Lenses 36 

"    23.  Simple  Hypermetropic  Astigmatism 38 

"    24.  Simple  Myopic  Astigmatism,    .   .           38 

"    25.  Compound  Hypermetropic  Astigmatis:\i,    ....  39 

"    26.  Compound  Myopic  Astigmatism, 39 

"    27.  Mixed  Astigmatism, 40 

"    28.  Refraction  and  Reflection  of  Light,     ....  48 

"    29.  Forms  or  Varieties  of  Lenses, 50 

"    30.  Convex  Lens  (Action  of), 51 

"    31.  Concave  Lens  (Action  of), 51 

"    32.  Plano-Prism  Lens  (Action  of) 52 

"    33.  Plano-Cylindrical  Lenses  (Action  of) 53 

"    34.  Lens  Grinders'  Lathe,  ....                 56 

"    35.  Tools  for  Grinding  Spherical  Lenses 57 

"    36.  Tools  for  Grinding  Cylindrical  Lenses,    ...  57 
"    37.  Photographs    of    Ancient  and   Modern  Spec- 
tacles   68 


INTRODUCTION. 


Thk  object  of  this  lecture,  as  indicated  by  the  title,  is  to 
place  such  facts  in  your  possession  that  will  1)etter  enaljlc 
you  to  assist  nature  in  the  care  of  your  eyes,  and  to  pay 
proper  attention  to  the  causes  that  may  tend  to  the  arrest 
and  prevention  of  impairment  and  loss  of  vision. 

We  all  appreciate  that  vision  is  the  most  valuable  of  the 
special  senses  ;  it  is  the  choicest  gift  of  nature.  By  it 
we  derive  more  knowledge  of  the  world  about  us  than 
through  any  other  medium,  and  yet  there  is  not  an  organ 
which  is  more  al)used  than  the  eye.  Without  doubt  the 
most  unfortunate  affliction  that  can  befall  any  one  is  the 
loss  of  his  eye-sight.  Although  it  is  of  such  great  impor- 
tance to  us  all,  yet  there  are  comparatively  few  persons 
who  have  any  definite  ideas  concerning  the  eye,  either  as 
to  its  structure  or  as  to  the  manner  in  which  it  aids  in  the 
production  of  sight. 

The  common  opinion  that  the  human  eye  is  theoretically 
perfect  is  erroneous  ;  indeed,  it  is  so  imperfect  that  it  led 
Helmholtz  to  remark  :  "  It  is  not  too  much  to  say  that 
if  an  optician  wanted  to  sell  me  an  instrument  which  had 
all  the  defects  of  a  human  eye,  I  should  feel  myself  quite 
justified  in  blaming  his  carelessness  in  the  strongest  terms, 
and  giving  him  back  his  instrument." 

However,  the  various  adjustments  by  which  the  defects 
are  neutralized  and  compensated,  overcome  the  faulty 
conditions  to  a  great  degree,  and  enable  the  organ  to  fulfill 
all  the  ordinary  requirements  which  may  be  made  upon  it. 

7 


In  fact,  the  author  is  happy  to  state  that  many  of  the  de- 
fects that  nature  has  unprovided  for,  have  been  remedied 
b}^  art. 

Before  beginning  any  Hfe-work  requiring  the  use  of  the 
eyes,  we  should  know  and  have  the  assurance  of  one  who 
is  competent  to  judge  that  the  organs  of  sight  are  in 
proper  working  condition.  Before  sending  a  child  to 
school  its  parents  and  guardians  should  have  his  eyes 
thoroughly  examined  in  order  to  ascertain  whether  they 
are  capal)le  of  performing  the  amount  of  work  that  may 
be  required  of  them.  Many  are  the  instances  where  a  child 
has  been  thought  stupid,  wdien  in  reality  the  entire  fault 
has  been  "defective  vision,"  which  when  properly  cor- 
rected, wrought  an  entire  change  in  the  demeanor  of  the 
child. 

The  eye  is  so  delicately  adjusted  that  if  heed  were  paid 
its  warnings  very  little  harm  would  result,  but,  unfor- 
tunately, everyone  seems  to  think  that  at  all  times  and 
under  all  physical  conditions  this  little  organ  can  be  kept 
at  high  pressure  ;  they  never  stop  to  think  that  injury 
or  disease  ma}'  lead  to  blindness  which  can  be  directly 
attributed  to  carelessness  and  neglect. 

In  the  following  pages  a  concise  description  of  the  eye 
itself,  with  an  explanation  of  its  mode  of  action  in  health, 
together  with  a  brief  outline  of  the  abnormal  conditions 
which  physicians  are  called  upon  to  treat,  may  be  found. 


THE  HUMAN  EYE. 

Part  I. 

For  sake  of  elucidation  and  comparison,  the  e^^e  ma}'  be 
likened  to  a  photographic  camera  ;  in  fact,  the  eye  and  the 
camera  resemble  one  another  in  man}^  respects,  and  to  a 
casual  o1)server  they  seem  to  be  identical  in  construction, 
but  as  a  practical  instrument,  the  superiority  of  the  former 


THE  HUMAN  EVE. 


i'lgure  I. 

A.     Sclerotic  Coat.  B.     Chorioid  Coat. 

D.     Refractive  Media.     (Lens  System.)  E.     Ii 

F.     Ciliarv  Muscle. 


C.     Retina. 
^.     (Uiapliragm.) 


over  the  latter  is  quite  evident.  The  former  (figure  i)  is 
a  masterpiece  of  nature,  and  its  mechanism  cannot  be 
imitated  by  man  ;  the  latter  (figure  2)  is  but  the  handi- 
work of  human  art. 

In  the  examination  and  study  of  the  organ  of  vision 
some  .systematic  plan  should  be  followed.  Before  describ- 
ing the  eyeball  it  is  well  to  become  acquainted  with  the 

9 


exterior  portion  of  those  parts  surrounding  the  globe — z.  <?., 
the  appendages  of  the  e^^e,  before  considering  the  eye 
proper,  from  without  inward. 

The  appendages  of  the  eye  inchide  : 


I.  The  Eyebrows.  V. 

II.  Tlie  EyeHds.  V. 

III.  The  P:yelashes.  VII. 

rV.  The  Orbit.  Vin. 


The  Conjunctiva. 
The  Lacrymal  Apparatus. 
The  Meibomian  Glands. 
The  Kxtrin.sic  Muscles. 


I.   Eyebrows   or   vSupercilia  (figure  3)  are  two    arched 
prominences  composed  of  short,  thick  hairs  that  are  di- 


TMK   PHOTOGRAPHIC  CAMKKA. 


Figfure  2. 

A.     Outside  wood  case.  l'>.     Inside  or  bellows  (black). 

C.     Sensitive  film  or  plate.  D.     Lens.  K.     Diai)liragni. 

!•".     Rack  and  Pinion,     (("onipare  with  similar  letters  in  ligiue  i.) 

rected  obliquely  up  and  out  from  the  median  line.  Their 
function  is  to  protect  the  organ  from  injury  and  to  pre- 
vent any  perspiration  that  may  trickle  down  the  forehead 
or  other  foreign  material  from  entering  the  eyes.  They 
are  also  useful  to  shade  the  eyes  from  excessive  light.  In 
addition,  they  have  aesthetic  functions,  being  powerful 
organs  of  expression  ;  for  example,  a  frown  is  produced 
by  wrinkling  or  depressing  the  brows,  whilst  by  elevating 
them   surprise  or  contempt  is  expressed  almost  as  plainly 


as  by  words.     Heav}^  e^^ebrows  and   long  eyelashes  are 
considered  beautiful,  and  are  very  much  admired. 

II.  Eyelids  or  Palpebra  (figures  3.  4  and  5).  Two 
thin  folds  placed  in  front  of  the  eye.  The  protective  cov- 
ering of  the  eyeball  consists  of  skin,  muscles  and  glands. 
The  upper  lid  is  the  larger,  and  is  the  more  movable  of 
the  two.  Besides  shielding  the  eye  from  injury,  the  lids, 
by  their  constant  motion,  moisten  and  lubricate  their  inner 
surfaces  and  the  anterior  portion  of  the  eyeball,  thus  en- 
abling   them    to    glide    easily   and    without  friction,   the 


FRONT  VIEW  OF  EYE. 


Pupil. 


Iris. 
5- 


Figure  3. 

3.     Sclerotic.         4. 
Supercilia  or  Eyehnn 


Prilpehra  or  eyelids. 


polish  and  transparency  of  the  cornea  being  thus  main- 
tained. Figure  4  shows  a  vertical  sectiim  of  the  upper 
e3^elid 

III.  Eyelashes  or  Cilia  (figure  4 )  act  as  sentinels,  guard- 
ing the  organ  against  injury  from  foreign  particles.  They 
project  in  two  or  three  rows  from  the  outer  edge  of  the  mar- 
gin of  each  lid.  Those  of  the  upper  lid  are  the  more  abun- 
dant, the  thicker  and  longer,  and  are  curved  downward 
and  outward.     In  the  lower  lid  the  cilia  are  less  numerous, 


smaller,  and  are  curved  upward  and  outward.  The)'  un- 
dergo a  constant  renewal,  each  lash  reaching  maturity  in 
five  or  six  weeks'  time.  It  then  drops  out,  and  is  suc- 
ceeded b}^  a  new  one.  Frequently  a  young  hair  may  be 
seen  projecting  by  the  side  of  an  old  one,  ready  to  take 
upon  itself  the  duty  for  which  it  has  been  called  into  ex- 
istence. 

VERTICAL  SECTION  Ul~  EVELID. 


Fi,<;iire  4. 

A.     f )ibirulaiis  muscle.      B.     Small  hair  in  skin  of  lid.       C.     Sweat  gland. 

I).     I'Ael.'ishes  — in  tlie  lower  row  can  be  seen  where  a  \oung  lash  is  just  hchiii 

and  ready  to  take  tlie  place  of  an  old  one. 

E.     Conjunctiva — lining  of  the  inner  surface.  F.     Meibt)niian  gland. 

Ct.     Excretorv  duct  at  the  border  of  the  lid. 


IV.  The  Orbit  (figure  7 )  is  the  cavity  under  the  fore- 
head, formed  by  the  bones  of  the  skull.  It  is  conical  in 
shape  with  its  ba.se  directed  forwards  and  outwards.  Its 
bony  walls  are  quite  thin,  but  its  edges  aie  comparatively 
strong,  particularly  the  upper  one,  which  is  more  promi- 
nent and  overhangs  the  e}'e,  thus  well  adapting  it  for 
.shielding  the  organ  from  injury.    Between  the  orbital  walls 


and  the  e3eban  there  is  a  padding  of  fat,  which  fills  the 
interstices  between  the  muscles,  nerves  and  blood  vessels, 
thus  protecting  the  parts  from  harm  ;  also  facilitates  the 
movements  of  the  eyeball,  thus  insuring  a  most  extensive 
and  free  range  of  vision. 

V.  The  Conjunctiva,  or  connecting  membrane  (figures 
4  and  5),  is  a  delicate,  highly  sensitive  mucous  membrane, 
connecting  the  eyelids  with  the  eyeball,  being  continuous 
with  the  skin  at  the  margin  of  the  lids.     It  covers  the 


POSTP^KIOk  roRTIOX  OF  EYELIDS. 
(RIGHT  EYE.) 


Figure  5. 

Showiiicr  relative  position  of  Lacrynial  Gland.     The  Meibomian  Glands  present 
the  appearance  of  parallel  strings  of  pearls. 

iinier  surfaces  of  the  lids  and  is  then  reflected  over  the  an- 
terior portion  of  the  globe  of  the  eye. 

VI.   Lacrymal  apparatus  (figures  5  and  6)  consists  of: 
a.  The  lacrymal  glands.  d.  The  lacr3mial  canals. 

d.   The  lacrymal  ducts.  e.   The  lacrymal  sac. 

c.    The  lacrymal  puncta.  /.    The  nasal  duct. 

a.  The  lacrymal  glands  are  the  glands  that  secrete  the 
tears.     Their  composite  size  is  about  that  of  an  almond. 

13 


b.  The  lacrymal  ducts.  These  number  from  seven  to 
ten ;  they  are  used  to  carry  tears  to  the  conjunctival 
surfaces. 

c.  The  lacrymal  puncta.  These  consist  of  minute 
orifices  that  can  be  seen  on  the  margin  of  the  lids.  They 
serve  as  the  beginnings  of  the  lacrymal  canals. 

d.  The  lacrymal  canals.     These  are  two  in  number,  the 

LACRYMAL  APl'ARATUS. 


Liicrymal  Gland  situated  at  the  upper  and  outer  jxulic 
Nasal  duct  situated  at  the  inner  and  lower  part  of  eve. 


Upper  and  the  lower.  They  are  intended  to  carr}^  off  the 
tears  after  having  served  the  purpose  of  washing  away 
any  substance  that  may  have  been  lodged  on  the  con- 
junctival surface.  When  there  is  an  excess  of  tears,  as 
during  weeping,  the  canals  cannot  carry  the  tears  away 
quickly  enough,  consequent! \-  the  tears  run  over  and 
trickle  down  the  cheek. 


e.  The  lacrymal  sac.  This  is  an  enlargement  of  the 
upper  portion  of  the  nasal  duct. 

f.  The  nasal  duct.  This  consists  of  a  membraneous 
canal  that  is  situated  in  the  nose.  It  begins  at  the  lower 
portion  of  the  lacrymal  sac  and  terminates  by  a  somewhat 
expanded  orifice  that  is  shaped  like  a  trumpet  end. 

VII.  The  Meibomian  Glands  (figures  4  and  5).    These 

EYEBALL  IN   PUSlT10x\  IN  ORBITAL  CAVITY. 


Figure  7. 

Extrinsic  Muscles  of  the  Eye. 

A. 

External  Rectus  Muscle.           B.     Superior  Rectus  Muscle. 

C. 

Inferior  Rectus  Muscle.             D.     Inferior  Oblique  Muscle 

E. 

Superior  Oblique  Muscle.         F.     Levator  of  upper  lid. 

are  the  sebaceous  glands  of  the  eyelid.  They  are  situated 
upon  the  inner  surface.  On  everting  the  eyelid  they  can 
be  distinctly  seen  through  the  mucous  membrane  present- 
ing the  appearance  of  parallel  strings  of  pearls,  or  appear- 
ing like  currants  on  a  stem,  each  having  an  excretory  duct 
at  the  free  border  of  the  eyelid  between  the  rows  of  cilia. 
Their  secretion  acts  as  a  lubricant,  preventing  adhesion  of 

IS 


the  lids  and  not  allowing  the  tears  from  coming  in  contact 
with  the  skin  and  thus  running  down  the  cheeks. 

VIII.  Muscles  of  the  Eye  (figure  7).  The  extrinsic 
nuiscles  are  attached  to  the  globe,  and  each  eye  is  provided 
with  its  own  set  of  muscles.  The  muscles  act  in  unison, 
liy  this  arrangement  the  e^^es  are  capable  of  being  directed 
simultaneously  to  any  object  which  it  may  be  desired  to 
view. 

There  are  six  muscles,  and  their  action  is  as  follows  : 

External  Rectus  Muscle,  moving  the  eye  outwards. 

Internal  Rectus  Muscle<  moving  the  eye  inwards. 

vSuperior  Rectus  Muscle,  moving  the  eye  upwards  and 
inwards. 

Inferior  Rectus  Muscle,  moving  the  e3'e  downwards  and 
inwards. 

Superior  Oblicpie  Muscle,  moving  the  eye  downwards 
and  outwards. 

Inferior  Obli(jue  Muscle,  moving  the  eye  upwards  and 
outwards. 


16 


THE  HUMAN  EYE. 


Part  II. 

The  form  of  the  human  eyeball  (figure  8)  is  that  of  a 
spheroid  about  one  inch  in  diameter,  having  the  segment 
of  a  smaller  sphere  engrafted  on  its  anterior  surface.  The 
anterior  face  of  this  smaller  sphere  projects  forward,  as  can 
readily  be  seen  in  looking  at  any  eye  from  the  side. 

The  smaller  sphere  is  formed  b}^  the  transparent  cornea, 
the  larger  one  consists  of  the  sclerotic  coat. 

THM     HIMAX    EVE 


Figure  8. 


Vertical  section,  showing  the  lelative  arrangement  and  positions  of 
the  various  parts  of  the  human  eye. 

The  eye  has  three  coats  or  tunics  : 
I.  Sclerotic.  II.   Chorioid.  III.   Retina. 

The  refractive  media  or  transparent  portions  of  the  eye 
are : 

A.  Cornea.  C.  Crystalline  Lens. 

B.  Aqueous  Humor.  D.   Vitreous  Humor. 

17 


I.  The  Sclerotic  Coat  (figures  i,  3  and  8)  is  the  thickest 
tunic  of  the  eyeball,  and  forms  "  the  white  of  the  eye." 
It  is  a  tough  and  elastic  opaque  membrane,  constituting 
five-sixths  of  the  diameter  of  the  globe,  the  other  sixth 
being  formed  by  the  cornea.  It  is  well  calculated  to  give 
shape  to  the  organ  and  offer  protection  to  the  more  delicate 
parts  contained  within.  It  also  serves  as  an  attachment 
for  the  muscles  that  move  the  eyeball. 

II.  The  Chorioid  Coat  (figures  i  and  8)  is  the  second 
or  middle  tunic  of  the  eyeball.  It  lies  between  the 
sclerotic  coat  and  the  retina.  It  is  quite  thin,  forming 
the  vascular  coat  of  the  organ,  and  contains  many  minute 
tortuous  bloodv^essels.  It  serves  as  a  nutritive  organ  for 
the  other  coats.  On  its  inner  surface  it  is  covered  with  a 
dense  layer  of  black  pigment  which  absorbs  all  excess  of 
light  that  falls  upon  it,  thus  preventing  internal  reflection 
which  otherwise  by  reflection  and  diffusion  would  prevent 
accurate  vision.  This  coat  has  been  likened  to  the  black- 
ened inner  surface  of  a  photographic  camera.  It  is  some- 
times absent,  and  when  this  is  the  case,  as  in  albinos, 
there  is  con.sideral)le  suffering  from  the  dazzling  effect  of 
the  light,  vision  always  being  below  the  normal. 

III.  The  Retina  (figures  i,  8,  9  and  10)  is  the  terminal 
expansion  of  the  optic  nerve  within  the  globe.  It  is  an 
exceedingly  complex  and  intricate  structure,  and  when 
examined  microscopically  is  found  to  consist  of  several 
layers.  It  is  the  nervous  portion  of  the  visual  organ.  It 
is  the  part  on  which  the  pictures  of  external  objects  are 
received  and  thence  transmitted  through  the  optic  nerve 
to  the  brain.  In  health  it  is  a  very  delicate,  transparent 
membrane.  Not  all  portions  of  the  retina  are  equally  sen- 
sitive. The  macula  lutea  or  yellow  spot  is  the  seat  of  the 
greatest  acuity  of  vision,  and  here  are  formed  the  sharpest 
and  most  distinct   images.     In   order  to  obtain   the  best 

18 


sight,  our  eyes  are  instinctively  directed  in  such  a  way  so 
that  the  rays  coming  from  an  object  fall  exactly  on  this 
portion  of  the  organ.  The  rods  and  cones  of  the  retina 
are  generally  considered  to  be  the  perceptive  layer. 

A.  Cornea  (figure  8)  is  a  transparent,  highly  polished 
membrane  forming  the  anterior  portion  of  the  eyeball.  It 
occupies  about  one-sixth  of  the  diameter  of  the  globe.     It 

RETINA. 


w^^K^  \im 

;  ^^^,^=0=..^ 

■    t'Aerviffa/  edtfc    f          I 

M^:.>,,.  1 

1             '.^-^^''''"^^"''fy^          <^^"f7  rot\c         1 
1          '^-^    --^-^^C=^^"'"'"                          i 

Fig:iire  9. 

Eye  ground  as  seen  by  an  examination  with  the  ophthalmoscope. 

Tlie  circular  plate  in  the  centre  of  the  illustration  is  the  optic  disc. 

ight  branches  "A"  emanating  from  the  centre  are  the  arteries  of  the  retina 

The  dark  branches  "  V"  going  to  and  dipping-  down  into  the 

centre  are  the  retinal  veins. 


fits  into  the  sclerotic  coat  very  much  as  a  watch  crystal  is 
inserted  into  the  rim  of  a  watch  case.  Although  it  appears 
structureless,  yet  under  the  microscope  it  is  found  to  con- 
sist of  five  layers  composed  of  cells  and  granules.  It  is 
through  this  membrane  that  the  color  of  the  eye  (the  iris) 
is  seen.  Were  it  not  for  the  eyelids  and  the  e3'elashes  the 
cornea  would  be  ver}-  prone  to  injur}'. 


B.  The  Aqueous  Humor  (figure  S)  is  a  clear,  watery 
fluid  which  occupies  the  space  called  the  ' '  iVnterior  and  the 
Posterior  Chamber,"  which  is  situated  between  the  cornea 
and  the  crystalline  lens,  the  iris  separating  the  two  parts. 

C.  The  Crystalline  Lens  (figure  8)  is  a  perfectly  trans- 
parent, highly  polished,  refractive  body,  shaped  very 
much  like  a  double  convex  lens,  with  the  curve  of  its  pos- 

SECTION  THROUCiH  THE  RETINA. 


Fijfure  lo. 

Ill  all  there  are  ten  (to)  layers,  composed  of: 

I.     Internal  litnititij^  niemhraiie.  2.     Fibrous  la>  er.  3.     La\er  of  vesicles. 

4.     Molecular  layer.         5.     Inner  nuclear  layer.         6.     Outer  molecular  layer. 

7.     Outer  nuclear  layer.  R.     Externallimitino;  membrane. 

9.     Layer  of  rod  and  cones.         10.     Pigmentary  layer. 

terior  surface  somewhat  greater  than  that  of  its  anterior. 
It  is  supported  between  the  iris  and  the  vitreous  humor  b}' 
a  thin,  transparent,  elastic  capsule.  The  lens  is  capable 
of  having  the  convexity  of  its  surfaces  increa.sed  by  the 
ciliary  muscle,  which  change  is  of  the  utmost  importance, 
since  it  enables  us  to   "accommodate" — that  is,  to  see 


near  objects  equally  well  as  distant  ones.  Normally,  the 
crystalline  lens  is  as  clear  as  the  purest  crystal,  hence  its 
name  In  the  later  years  of  life  the  lens  becomes  more  or 
less  cloud}^  giving  rise  to  the  condition  known  as  Cata- 
ract. 

D.  The  Vitreous  Humor  (figure  8)  is  a  transparent,  col- 
orless, gelatinous  mass,  resembling  molten  glass.  It 
occupies  about  four-fifths  of  the  cavity  of  the  globe.  It 
contains  a  depression  or  hollow  in  front  for  the  reception 
of  the  crystalline  lens.  Its  principal  functions  are  to  aid 
in  bringing  rays  of  light  to  an  accurate  focus  on  the  retina, 
to  contribute  to  the  solidity  of  the  eye,  and  to  hold  the 
retina  in  proper  place. 

The  Iris  (figures  3  and  8)  is  a  thin,  muscular  curtain 
which  is  suspended  in  front  of  the  crystalline  lens.  It 
serves  the  purpose  of  a  diaphragm,  thus  cutting  off  all 
superfluous  light  and  correcting  the  spherical  aberration 
which  is  present  in  every  crystalline  lens.  It  is  composed 
of  radiating  and  circular  muscular  fibres.  The  former 
converge  from  the  circumference  towards  the  pupil,  which 
the}'  serve  to  dilate.  The  latter  encircle  the  pupillar}- 
opening,  and  in  response  to  their  action  the  pupil  becomes 
contracted.  It  is  the  membrane  that  gives  the  eye  its 
vSpecial  color,  and  upon  which  the  beauty  of  the  organ,  to 
a  great  degree,  depends.  It  is  perforated  in  the  centre, 
making  a  circular  opening  known  as  the  pupil.  During 
distant  vision  the  pupil  becomes  expanded  and  during  near 
vision  it  is  contracted. 

The  Pupil  (figure  3)  forms  the  "  black  of  the  eye.''  It 
is  a  hole  in  the  iris.  The  quantit}^  and  quality  of  the 
light  that  falls  upon  the  eye  regulates  in  measure  the  size 
of  the  pupil.  The  greater  the  amount  of  light  the  smaller 
the  pupil.  As  the  iris  expands  the  pupil  becomes  con- 
tracted.    This  can  be  verified  by  asking  some  one  to  face 

21 


the  bright  light  coming  through  a  window  and  noticing 
the  diminished  size  of  the  pupil,  and  then  having  the  same 
person  turn  their  back  to  the  window  and  notice  how 
quickly  the  pupil  enlarges.  It  is  for  the  same  reason  that 
in  entering  from  a  dark  to  a  light  room  one  feels  dazed 
until  the  pupil  can  adjust  itself  to  the  conditions  of  its  new 
surroundings. 

The  Ciliary  Muscle  (figure  8)  consists  of  a  great  num- 
ber of  delicate  bands  of  muscular  tissue,  which  form  a 
circle  around  the  edge  of  the  crystalline  lens.  Lying 
just  behind  the  iris,  although  small  and  insignificant  in 
appearance,  the  muscle  is  an  essential  part  of  the  eye, 
as,  by  its  action,  contracting — now  more,  then  less — the 
convexity  of  the  crystalline  lens  is  increased  or  dimin- 
i.shed,  according  to  the  necessity  of  the  moment ;  thereby 
focusing  all  external  ol^jects  accurately  on  the  retina. 


22 


NERVES  OF  THE  EYE. 

1.  The  Optic  Nerve  (figures  8  and  1 1 )  is  a  nerve  of  spe- 
cial sen.se  ;  "  the  sense  of  sight"  which,  on  entering  the 
globe,  expands  and  ])ecomes  the  retina. 

2.  Motor  Nerves,  that  help  control  the  various  move- 
ments of  the  eyeball. 

HASH  OF  THE   HUMAN  BRAIN. 


Figure  ii. 


The  optic  tracts  and  the  optic  chiasm  where  optic  nerves  cross 
just  before  entering  tlie  e>e. 

3.  Sensor}'  Nerves,  which  are  used  for  taking  cogni- 
zance of  impressions  received  from  the  external  source  by 
means  of  contact. 

ARTERIES  OF  THE  EYE. 

The  arteries  of  the  eye  supply  it  with  nourishment.  A 
portion   of  them  pa.ss  through  the  optic  nerve  .similar  to 

23 


the  plumbago  in  a  lead  pencil,  and  when  they  reach  the 
inner  portion  of  the  organ  divide  into  four  branches,  one 
going  to  each  quarter. 


VEINS  OF  THE    EYE. 

There  are  four  principal  veins  in  the  eye.     These  are 
used  to  carry  off  the  blood  after  it  has  served  its  purpose. 


PHYSIOLOGY  OF  VISION. 

A  number  of  conditions  are  essential  to  normal  vision, 
7.  e.,  The  ol^ject  which  is  desired  to  be  seen  must  be  illu- 
nnnated  so  that  the  rays  of  light  emanating  from  the  same 
will  reach  the  eye. 

The  axis  of  the  eye  must  be  directed  towards  the  object 
which  is  desired  to  be  seen. 

The  eye  must  be  capable  of  receiving  the  rays  which 
must  pass  unobstructed  from  the  o])ject  through  the  media 
to  the  retina. 

The  connection  which  exists  between  the  retina  and  the 
brain  must  be  in  a  normal  condition. 

The  visual  centre,  or  that  portion  of  the  brain  which  is 
allotted  the  sense  of  seeing,  must  be  capable  of  elaborating 
the  impressions  conve3^ed  to  it  into  what  is  termed  vision. 

The  dioptric  apparatus  is  composed  of  a  number  of 
lenses  possessing  a  high  condensing  power  ;  this,  which  is 
composed  of  the  cornea,  the  crystalline  lens,  the  aqueous 
and  vitreous  humors,  assist  in  bringing  the  rays  coming 
from  an  object  to  a  focus  on  the  macula  lutea  or  yellow 
spot  in  the  retina,  this  being  the  portion  of  the  retina 
which  receives  a  correct  impression  of  objects. 

24 


VISUAL  ANGLE. 

The  size  of  the  retinal  image  depends  upon  the  size  of 
the  visual  angle,  one  being  m  direct  proportion  to  the 
other,  the  nearer  the  object  is  brought  to  the  eye  the  greater 
being  the  visual  angle  and  the  larger  is  the  image. 

In  order  that  the  retinal  image  may  be  of  the  necessary 
size  to  excite  perception,  the  object  producing  it  must 
form  a  certain  visual  angle,  which  must  not  be  too  small. 

A  good  illustration  and  test  of  the  angle  of  vision  can 
be  made  by  placing  a  coin  near  the  eye  and  looking  to- 
wards some  distant  object.  If  the  coin  be  held  close 
enough  to  the  eye  it  will  obscure  the  far  object  from  view, 

VISUAL  ACUTENESS. 


Figure   12. 

The  letters  at  8  and  12  feet  are  as  plainly  seen  by  the  eye  as  the  one  at  4  feet ; 
the  lines  suhteiul  from  all  of  the  letters  at  the  same  angle. 

but  if  the  coin  be  removed  to  arm's  length  distance  it  will 
appear  much  smaller,  allowing  the  distant  object  to  be 
seen. 

VISUAL  ACUTENESS. 

In  ophthalmic  practice  the  acuity  of  vision  is  determined 
by  a  series  of  appropriate  letters  of  various  sizes  (figure  1 2), 
each  having  a  definite  standard  value  to  be  distinguished 
at  a  specified  distance  by  the  normal  eye.  Each  com- 
ponent stroke  of  the  letter  is  seen  under  an  angle  of  one 
minute,  the  entire  letter  being  enclosed  within  an  angle 
of  five  minutes. 

25 


As  the  lines  in  the  illustration,  diverging  from  the  eye, 
subtend  at  an  angle  of  five  minutes,  any  object  placed  in 
this  angle  can  be  readily  distinguished  by  an  eye  which 
has  normal  acuity  of  vision. 

The  letter  E  at  twelve  feet  is  three  times  higher  than 
the  one  of  four  feet.  Should  the  object  be  brought  nearer, 
the  visual  angle  would  be  greater,  and  consequently  a 
larger  retinal  image  would  be  the  result — if,  for  instance, 
an  object  the  size  of  the  letter  K  at  twelve  feet  were 
brought,  say  to  four  feet  or  nearer,  the  image  on  the  retina 
would  be  increased  and  the  object  would  be  seen  moredis- 

ACCOMMODATION. 


Figure  13. 

Rij^lil  half  is  represented  as  adjusled  for  distant  vision, 
and  left  half  for  near  vision. 


tinctl^^  If,  however,  the  letter  or  object  which,  under  the 
rule  can  only  l)e  seen  well  at  four  feet,  were  removed  to 
twelve  feet,  it  would  become  too  indistinct  to  recognize,  as 
the  retinal  image  would  be  too  small. 


ACCOMMODATION. 

The  term  as  used  in  this  connection  is  the  power  the  eye 
has  of  adjusting  itself  and  rendering  near  points  visually 
distinct.  It  is  a  semi-voluntary  action.  The  act  is  de- 
pendent upon  the  physiological  power  of  the  ciliary  muscle 

26 


and  the  inherent  elasticity  of  the  crystalline  lens.  This 
lenticular  mobility  is  greatest  in  youth,  gradually  dimin- 
ishing as  age  advances. 

Raj^s  coming  from  a  near  object  are  more  or  less  diver- 
gent, and  should  they  enter  a  normal  eye  when  it  is  in  a 
state  of  rest  (that  is,  adjusted  for  distant  objects),  the^^ 
would  be  focussed  at  a  point  behind  the  retina. 

What  then  must  take  place  in  order  to  bring  the  rays 
from  the  object  to  a  focus  on  the  retina  ?  The  refractive 
power  of  the  eye  must  be  increased,  this  being  accom- 
])lished  by  a  change  in  the  crystalline  lens,  which  has  been 

ACCOMMODATIOX. 


Figure  14. 

By  an  increase  in  the  convexity  of  the  crystalline  lens,  the  point  F,  at  which  the 

solid  lines  meet  behind  the  retina,  are  brought  to  a  focus 

at  point  F',  as  shown  by  dotted  line. 


shown  to  take  place  in  the  anterior  surface  of  the  lens, 
which  becomes  more  convex  and  approaches  the  cornea, 
the  posterior  surface  of  the  lens  also  becoming  a  trifle  more 
convex. 

This  change  is  affected  b}-  the  ciliary  muscle.  To  show 
that  this  does  take  place,  can  be  proven  by  holding  a  piece 
of  netting  or  gauze  twelve  or  fifteen  inches  in  front  of  your 
eyes,  and  fixing  3'our  gaze  intently  on  some  distant  object. 
As  long  as  the  distant  object  is  clearly  seen,  the  meshes  of 
the  netting  will  be  indistinct,  while  if  the  meshes  be  accu- 

27 


rately  seen  the  distant  object  will  be  obscure  and  no  longer 
plainly  visible  ;  in  other  words,  there  has  been  a  change 
in  the  convexity  of  the  lens,  this  being  the  greater  fornear 
and  the  less  for  distant  objects. 

The  alteration  in  the  curvature  of  the  crystalline  lens  is 
shown  in  figure  13,  which  represents  a  horizontal  section 
of  the  anterior  part  of  an  eye.  The  right  half  of  the  figure 
shows  the  eye  when  at  rest — that  is,  adjusted  for  distant 
objects.  The  left  half  represents  it  as  accommodated  for 
near  vision,  showing  an  increase  in  the  convexity  of  the 
crystalline  lens.  Figure  14  illustrates  how  an  increase  in 
the  thickness  of  the  lens  shortens  the  focus  of  rays,  which 
would  otherwise  come  together  behind  the  retina  and  not 
produce  a  ])roper  focus. 


28 


THE  EMMETROPIC  OR  NORMAL  EYE. 

When  an  eye  is  in  a  state  of  rest — that  is,  when  the  cil- 
iary nmscle  is  relaxed  to  its  fullest  extent  and  the  accom- 
modation suspended— the  rays  emanating  from  a  distant 
object  striking  the  convex  surface  of  the  cornea  in  parallel 
lines  and  in  conjunction  with  the  crystalline  lens,  and  the 
aqueous  and  vitreous  humors  being  brought  to  a  focus  on 

THE  EMMETROPIC  OR  NORMAL  EYE. 


Figure  15. 
Parallel  rays  focus  directly  on  the  retina  at  F. 

the  retina,  thus  producing  a  perfect  and  distinct  image 
without  any  artificial  aid,  we  have  a  "  Normal  Eye." 

The  emmetropic  eye  is  usually  about  one  inch  long  in 
its  anteroposterior  diameter,  5'et  it  ma}'  be  either  longer  or 
shorter  and  still  be  normal,  providing  that  if  it  be  the 
former  the  refraction  of  the  media  be  correspondingly  in- 
creased, and  if  it  be  the  latter  correspondingly  lessened. 


29 


HYPERMETROPIC  OR  FAR-SIGHTED  EYE. 

Hypermetropia  caused  by  an  anteroposterior  shortening 
of  the  globe,  may  be  congenital  or  acquired.  It  is  due  to 
a  faulty  shape  of  the  eyeball  or  to  a  too  little  strength  of 
the  refractive  media.  In  each  case  the  visual  angle  is  too 
long  and  the  rays  come  to  a  focus  behind  the  retina. 

The  sight  is  generalh^  good  for  distant  objects,  but  near 

HYPERMETROPIC  OR  FAR-SIGHTED  EYE. 


Parallel  rays  coining  to  a  focus  behind  the  retina  at  F, 
Dotted  line  shows  shape  of  normal  eye. 


objects  become  blurred  and  cannot  be  distinctly  seen  with- 
out undue  accommodative  effort.  The  eyes  cannot  be 
used  for  any  period  of  time  without  becoming  much  fa- 
tigued. The  mu.scular  effort  to  adjust  the  rays  on  the  retina 
is  too  great  and  the  ciliary  muscle  soon  relaxes,  causing 
the  object  looked  at  to  run  together,  leading  to  headache, 
asthenopia,  and  sometimes  strabismus.  Hypermetropes 
generally  complain  of  headache,  especially  over  the  eyes, 
and  quite  often  attribute  the  pain  over  and  in  the  eyes  to 


nervous  headache  ;  a  condition  that  would  be  soon  relieved 
by  proper  convex  glasses.  A  convex  lens  is  necessary  to 
bring  the  rays  to  a  proper  focus  on  the  retina.  (See 
figure  17.) 

HYPERMETROPIC  OR  FAR-SIGHTED  EVE. 


Figure  17. 

A  proper  convex  lens  placed  in  front  of  this  eye  will  shorten  the  rays  from 
F  to  F'  so  that  they  will  focus  exactly  on  the  retina. 


The  visual  angle  in  hypermetropic  eyes  may  be  so  long 
that  glasses  are  required  for  distance  to  obtain  clear  and 
distinct  vision  as  well  as  for  near  work.  In  young  hyper- 
metropes  the  ciliary  muscle  often  has  sufficient  power  to 
overcome  effects  of  a  moderate  amount  of  hypermetropia. 


MYOPIC  OR  NEAR-SIGHTED   EYE. 

In  myopia  parallel  visual  rays  are  brought  to  a  focus 
anterior  to  t'le  retina  (see  dotted  lines  in  figure  i8).  The 
condition  is  either  liereditary  or  acquired.  In  the  former 
it  is  generally  due  to  a  lengthening  of  the  anteroposterior 
optic  axis  or  a  state  of  too  great  a  convergence  of  the 
media,  while  in  the  latter  it  is  dependent  upon  prolonged 
strain  during  near  vision.  The  condition  is  either  pro- 
gressive or  stationary.     In  the  latter  it  is  generally  of  low 

MYOPIC  OR  np:ar-sighted  eve. 


Figure  i8. 

Parallel  rays  coming  lo  a  focus  at  F  before  tliey  reach  the  retina. 
Dotted  line  shows  shape  of  normal  eye. 


degree,  whereas  in  the  former  it  is  of  higher  grades,  which 
always  have  a  tendency  to  increase.  In  myopes  the  power 
of  observation  for  distant  objects  is  limited,  and  the  more 
they  accommodate  and  try  to  see,  the  more  blurred  be- 
comes the  image  of  the  distant  ol)ject. 

Treatment  is  merely  orthopedic  and  preventive.  The 
condition  cannot  l^e  altered.  Hygienic  methods  with  the 
correction  of  the  refractive  error  so  as  to  cause  the  myopia 


to  remain  stationary,  are  necessary.  Stooping  or  reading 
in  a  recumbent  posture  must  be  avoided.  Such  eyes  should 
not  be  used  at  dusk  or  by  bad  light. 


MYOPIC  OR  NEAR-SIGHTED  EYE. 


Figure  19. 

A  proper  coucave  lens  placed  in  front  of  this  eye  will  lengthen  the  rays  from 
F  to  F'  so  that  they  will  focus  exactly  on  the  retina. 


The  weakest  concave  glasses  which  enable  such  eyes  to 
unite  divergent  rays  directly  upon  the  retina,  should  be 
worn.  Myopes  who  continue  to  strain  their  eyes  will  only 
increase  their  trouble,  whereas  should  they  wear  suitable 
glasses  they  will  check  the  disease.  In  such  cases  a  con- 
cave lens  is  necessary  to  bring  rays  to  a  proper  focus  on 
the  retina.     (See  figure  19.) 


33 


ASTIGMATISM. 

Astigmatism  is  a  condition  of  an  eye  whose  curvature 
is  unequal,  "-he  radius  being  greater  in  one  direction  than 

TEST  CARD   FOR   ASTIGMATISM. 


Chart  used  for  the  detection  of  astigmatism.     To  a  normal  eye  all  the  lines 
should  appear  equally  distinct. 


in  the  other.  The  shape  of  the  eye  may  be  hkeiied  to 
that  of  the  bowl  of  a  spoon.  This  want  of  synmietr}^  in  the 
dioptric  apparatus  is  nearly  always  situated  in  the  cornea, 
but  similar  inequalities  may  also  exist  in  the  crystalline 
lens.  Although  this  defect  is  very  common,  yet  not  until 
the  last  few  years  has  its  frequency  and  importance  been 
fully  recognized.  Nearly  all  eyes  are  more  or  less  astig- 
matic, the  cause  usually  being  congenital,   but  the  con- 

34 


dition  may  be  acquired.  On  account  of  the  asymmetry  of 
the  refracting  surfaces,  it  produces  a  distorted  image  on 
the  retina,  which  disturbs  and  diminishes  the  acuity  of 
vision  according  to  the  degree  of  the  defect.  Both  distant 
and  near  vision  are  equally  affected,  since  at  no  point 
can  a  distinct  image  be  obtained.  Vision  is  not  only 
blurred  as  in  hypermetropia  and  myopia,  but  it  presents 
certain  peculiarities — a  sphere,  for  instance,  appearing 
elliptical  in  shape. 

TEST  CARD   FOR  ASTIGMATISM. 


Figure  21. 
Chart  illustrating  appearance  of  disc  in  astigmatism. 

c\nother  distinguishing  feature  of  this  defect  is  the  fact 
that  certain  groups  in  a  series  of  lines,  such  as  represented 
in  figure  20,  are  seen  with  more  distinctness  and  appear 
blacker  than  those  that  are  situated  in  the  opposite  merid- 
ian. For  this  reason  a  common  complaint  of  those  who 
are  aifected  with  the  condition  is  the  trouble  they  have  in 
recognizing  the  hands  of  a  clock  at  certain  hours. 

To  an  astigmatic  eye  a  chart  similar  to  that  shown  in 

35 


figure  20  appears  partly  blurred  as  the  one  shown  in  figure 
21.  For  example,  a  star  forms  an  image  upon  the  retina, 
which,  instead  of  being  seen  as  a  point,  will  appear  as  a 
line  or  an  oval.  The  meridia  of  the  greatest  and  the  least 
refraction  are  called  principal  meridians.  In  speaking  of 
the  principal  meridians  of  astigmatism,  the  vertical  and 
horizontal  are  usually  meant.  They  may,  however,  occur 
in  any  position,  and  they  are,  as  a  rule,  with  but  few  ex- 
ceptions, found  to  be  at  right  angles  to  one  another. 

Myopes  usually  see  the  vertical  set  of  lines  the  most 
distinctly  and  hypermetropes  the  horizontal  ones.     If  this 

CYLINDRICAL    LENSES. 


E.^ur.  22. 

A.  Convex  cylindrical  lens  used  for  tiie  correction  of  hypermetropic  astigmatism. 

B.  Concave  cylindrical  lens  used  for  the  correction  of  myopic  astigmatism. 

C.  Concavo-convex  cylindrical  lens  used  for  tiic  correction  of  mixed  astigmatism. 


be  reversed,  astigmatism  is  said  to  be  against  the  rule.  It 
is  not  uncommon  to  find  the  eye  faulty  in  the  oblique 
meridians.  Astigmatism  cannot  be  cured,  but  it  can  be 
corrected.  In  the  treatment  of  this  defect,  the  rays  of 
light  must  be  gathered  into  a  single  focus.  For  this  pur- 
pose recourse  must  be  had  to  properly  .selected  cylindrical 
lenses,  such  as  th(3se  that  are  represented  in  figure  22. 

Ordinary  spherical  lenses  will  not  answer  for  the  correc- 
tion of  astigmatism,  since  the  rays  passing  through  such 
lenses  are  refracted  equally  in   all   directions,  and  what  is 

36 


desired  is  to  find  a  lens  that  will  refract  or  collect  rays  in 
but  one  direction.  In  the  correction  of  astigmatism,  not 
only  must  the  proper  neutralizing  lens  be  found,  but  the 
angle  of  the  astigmatism  be  determined.  Correcting 
lenses  are  prescribed  and  ground  in  accordance  with  the 
formula  adapted  to  each  individual  case. 

The  selection  of  these  lenses  can  only  be  determined  after 
careful  and  repeated  examinations.  This  important  work 
should  only  be  entrusted  to  a  competent  physician.  No 
matter  how  slight  the  astigmatism,  it  should  be  corrected, 
as  it  is  the  prime  factor  of  more  headaches,  nervous  and 
functional  disorders,  than  all  other  ocular  errors  com- 
bined. Such  conditions,  if  permitted  to  go  unheeded,  in- 
capacitate the  person  for  mental  labor,  and  often  distract 
the  sufferer  to  such  a  degree  that  life  does  not  seem  worth 
living.  In  fact,  the  author  has  had  cases  on  the  verge  of 
insanity,  and  those  even  afflicted  with  epileps3\  in  which 
the  apparent  underlying  cause  was  astigmatism,  and  which 
were  cured  by  careful  treatment  and  properly  selected 
lenses. 

VARIETIES   OF  ASTIGMATISM. 

1.  Simple  Hypermetropic  Astigmatism. 

2.  Simple  Myopic  Astigmatism. 

3.  Compound  Hypermetropic  Astigmatism. 

4.  Compound  Myopic  Astigmatism. 

5.  Mixed  Astigmatism. 

6.  Irregular  Astigmatism. 

I.  In  simple  hypermetropic  astigmatism,  the  rays  com- 
ing through  one  of  the  principal  meridians,  which  in  this 
case  is  normal,  focus  exactly  on  the  retina,  while  those 
coming  through  the  meridian  at  right  angles  to  the  former 
focus  beyond  the  retina.     See  figure  23. 

37 


2.  Ill  simple  in3'Opic  astigmatism,  the  rays  going  through 
one  of  the  principal  meridians,  which  is  emmetropic,  focus 
exactly  on  the  retina,  while  those  going  through  the  other 
principal  meridian  fall  short  of  the  retina.     See  figure  24. 

simplp:  tivpermetropic  astigmatism. 


Figure  23. 
Vertical  ineridian  is  normal.      Horizontal  meridian  is  hyi)ermetropic. 

3.  In  compound  hypermetropic  astigmatism  the  ra3^s  are 
too  long  and  are  projected  beyond  the  retina  through  both 
the  principal  meridians,  the  point  of  focus  however  in  one 
direction  being  behind  that  of  the  other.     See  figure  25. 

SIMPLE  MYOPIC  ASTIGMATISM. 


Figure  .'i. 
Horizo:.tal  meridian  is  normal.     Vertical  meridian  is  myopic 


4.  Ill  compound  myopic  astigmatism,  the  rays  passing 
through  both  principal  meridians  fall  short  and  are  focused 
before  they  reach  the  retina  ;  those  in  one  direction  com- 

3'S 


ing  to  a  focus  more  quickly  than  those  situated  at  right  or 
opposite  angle.     See  figure  26. 

5.   Mixed  astigmatism,  as  the  term  suggests,  is  a  mix- 
ture of  the  conditions  found   in  the  first  and  the  second 

COMI'orXl)  HYPERMETROPIC   ASTIGMATISM. 


iMgure  25. 
15oih  iiiei  idiaiis  are  hypermetropic.        The  horizontal  is  more  so  than  the  \ertical. 

varieties.  The  person  is  hypermetropic  in  one  direction 
and  myopic  in  the  other.  The  rays  coming  through  one 
principal  meridian  are  brought  to  a  focus  before  they  reach 
the  retina,  and  the  rays  coming  in  at  the  opposite  meridian 

rOMPOUXD  MYOPIC   ASTIGMATISM. 


Figure  26. 
Both  meridians  are  myopic.      The  vertical  more  so  than  the  horizontal. 


are  brought  to  a  focus  beyond  the  retina.     See  figure  27. 

6.   Irregular  astigmatism  is  the  condition  in  which  there 

are  several  refractive  errors  in  the  same  meridian.     It  may 

be  either  congenital  or  acquired.     It  is  almost  impossible 

39 


to  correct  these  cases  by  lenses,  although   the}^  may  be 
improved  to  some  extent,  especially  if  a  stenopeic  slit  set 


MIXED    ASTIGMATISM. 


Figure  27. 
Vertical  meridian  is  myopic  and  the  horizontal  is  hypermetropic. 

at  the  best  meridian  (in  order  to  cut  off  all  other  rays)  is 
used,  thus  avoiding  any  metamorphopsia   that  may  arise. 


40 


PRESBYOPIA  OR  OLD  SIGHT. 

Presbyopia,  "old  sight,"  is  an  accompaniment  of  the 
later  3'ears  of  life.  It  is  a  ph^^siological  or  natural  change, 
and  affects  all  eyes.  The  condition  is  dependent  almost 
solely  upon  the  failure  of  the  accommodation,  this  being 
due  to  a  gradual  hardening  of  the  crystalline  lens,  render- 
ing it  incapable  of  increased  convexity,  and  to  a  decrease 
of  the  power  of  the  ciliary  muscle. 

Although  this  decrease  in  the  power  of  adjustment  for 
near  objects  is  not  evidenced  until  perhaps  the  fortieth  or 
forty-fifth  3^ear  of  life,  yet  the  change  has  been  gradually 
taking  place  for  years,  and  accommodation  has  been  grow- 
ing increasingly  weaker.  Fine  print  or  near  objects  can 
no  longer  be  seen  distincth'  at  this  period  of  life. 

Presbyopia  is  assumed  when  there  is  difficulty  in  read- 
ing fine  print  within  eight  inches.  The  first  symptom 
usually  noticed  is  that  it  is  difficult  to  distinguish  fine  type, 
and  the  work  must  be  held  further  away  from  the  eyes  and 
more  strongly  illuminated  in  order  to  obtain  distinct  vision. 
These  symptoms,  which  at  first  are  pronounced  by  artifi- 
cial light,  will  also  later  manifest  themselves  in  the  day- 
time. 

In  the  treatment  of  this  condition  convex  lenses  are 
required  to  restore  the  near  point.  It  is  useless,  by  strain- 
ing the  eyes,  to  attempt  to  postpone  the  use  of  glasses. 
The  popular  opinion  is  to  do  so,  but  it  is  a  great  mistake. 
The  longer  the  eyes  are  deprived  of  the  aids  of  which  thej^ 
are  in  need,  the  more  rapidly  will  the  senile  changes  in  the 
eye  become  developed.  In  old  persons,  when  distant 
vision  becomes  indistinct,  glasses  of  proper  strengths 
should  be  worn  constanth'. 

41 


HYGIENE   OF  THE   EYE. 

Direction  and  vSource  of  light  are  of  great  importance, 
especially  in  doing  near  work.  The  ideal  light,  or  that 
which  is  softest  and  most  pleasant  to  the  eye,  is  the  dif 
fused  light  from  a  northern  sky.  Next  to  diffused  da3dight, 
properly  located  incandescent  electric  lamps  give  the  best 
light,  because  they  burn  steadier  and  radiate  less  heat 
than  the  burners  which  depend  for  their  vitality  upon  the 
surrounding  atmosphere.  Gas  and  oil  lamps  consume 
more  or  less  of  the  oxygen  in  the  air,  and  there  is  always 
a  tendency  to  rub  the  eyes  after  prolonged  use,  l)ecause 
they  feel  dry  and  hot,  on  account  of  the  air  becoming 
heated  and  vitiated. 

An  axiom  in  good  artificial  light  is  to  keep  the  illumin- 
ation of  objects  as  strong  as  possible,  but  the  intensity  or 
brilliancy  of  lights  as  low  as  may  be  compatible  for  good 
vision.  By  all  means  shades  should  be  employed  over 
lamps  in  order  to  protect  eyes  from  the  direct  rays  of  the 
light.  It  is  well  to  intercept  the  rays.  Opal,  opaline,  or 
ground-glass  shades  can  be  used  for  this  purpose,  though, 
unfortunately,  the}^  waste  from  thirty  to  sixty  per  cent,  of 
the  light.  The  7'e?y  besf  method  for  effecting  the  better 
diffusion  and  distribution  is  accomplished  by  the  use  of 
"The  Holophane  Glass  Globe"  placed  around  the  source 
of  illumination.  This  is  a  system  of  compound  prisms  in 
which  the  very  finest  transparent  glass  is  used.  In  conse- 
quence, the  light  is  intensified  and  at  the  same  time  sof- 
tened and  diffused.  By  varying  the  angles  of  the  prisms 
of  which  the  glolies  are  composed  the  light  can  be  directed 

42 


over  any  desired  point  or  space.  Each  facet  is  so  arranged 
by  carefully  calculated  measurements  that  the  whole 
surface  of  the  globe  becomes  softly  luminous. 

When  artificial  illumination  must  be  resorted  to,  choice 
must  be  made  for  the  best  that  can  be  procured,  no  matter 
what  the  source  may  be.  It  should  be  steady  and  direct, 
rather  than  that  from  a  reflected  or  a  flickering  light.  Light 
should  enter  from  above  and  at  the  side,  then  to  pass  over 
the  left  shoulder,  in  order  that  it  will  strike  the  page  of 
the  book  or  the  work.  The  aim  should  be  to  have 
the  object  which  is  desired  to  view  thoroughly  illumin- 
ated, and  at  the  same  time  the  eyes  properly  shaded.  The 
importance  of  sufficient  light  is  made  quite  manifest,  if  an 
attempt  be  made  to  read  in  a  dimly  lighted  room,  or  in 
twilight.  The  work  is  brought  nearer  to  the  eyes  in  order 
to  secure  a  larger  retinal  image  and  to  obtain  increased 
illumination.  The  consequent  strain  upon  the  accommo- 
dation and  convergence  brought  about  b}-^  this  abnormal 
near  point  soon  produces  undue  congestion  of  the  eyeball 
and  surrounding  tissue,  and  thus  leads  to  increased  intra- 
ocular tension,  with  spasm  of  accommodation,  resulting  in 
headache  and  other  nervous  symptoms.  Too  much  light, 
especially  if  it  be  reflected,  is  injurious,  as  it  produces  an 
overstimulation  of  the  retina.  Sudden  changes  from  dark- 
ness to  light,  and  vice  versa,  siiould  be  shunned.  The  eyes 
should  not  be  dazzled  by  the  light  reflected  from  white 
paper,  snow,  water,  or  in  fact,  any  polished  surface. 

There  is  no  doubt  that  deficient  and  improperly  ad- 
mitted light  in  school  rooms  is  one  cause  of  the  rapid  prog- 
ress of  optical  defects,  especially  myopia.  In  the  first 
plnc2,  the  desk  should  never  be  arranged  so  that  the  pupil 
faces  the  window.  To  sit  facing  a  light  during  study  is 
extremely  injurious  to  the  best  of  eyes.  On  looking  up, 
the  eye  becomes  saturated  with  light,  and  then  on  turning 

43 


to  the  printed  page,  an  extra  accommodative  effort  must 
be  made  to  overcome  the  dazzle,  and  clear  up  the  vision. 

School  furniture  is  often  ill  adapted  for  the  scholar,  even 
if  it  be  properly  placed  in  regard  to  light.  The  bench  is 
often  too  high  for  the  desk,  so  that  the  pupil  must  bend 
over  his  work,  thus  favoring  congestion  to  the  head  and 
contributing  to  the  congested  condition  at  the  back  of  the 
eyes.  Often  the  seat  is  too  far  away  from  the  desk,  the 
head  is  thereby  brought  too  near  the  book,  so  that  the  de- 
velopment of  nearsight  is  directl}^  encouraged. 

It  is  of  great  importance  that  the  desks  and  the  seats 
should  be  of  the  proper  height  and  angle.  The  desk 
should  have  a  slight  downward  slope.  The  arrangement 
of  the  same  must  be  such  that  the  student  will  prefer  the 
correct  position  rather  than  assume  one  that  is  abnormal 
although  a  more  comfortable  attitude.  This,  of  course,  also 
applies  to  "office"  desks  and  illumination.  This  may 
seem  trivial,  but  when  it  is  considered  that  from  five  to  six 
hours,  or  even  longer,  are  spent  each  day  in  near  work,  it 
is  advisable  to  give  this  subject  considerable  thought  and 
attention.  In  making  a  selection  of  books,  if  possible, 
always  choose  those  that  have  good  unglazed  paper,  with 
large  and  clear  type.     Tinted  paper  is  the  most  restful. 

Reading  should  never  be  attempted  while  lying  down, 
or  when  in  a  reclining  posture.  Many  a  tedious  case  of 
weak  sight  has  been  traced  to  the  pernicious  habit  of 
reading  in  bed  after  retiring  for  the  night.  What  is  prob- 
ably as  bad,  or  worse,  and  a  ver}^  common  way  of  strain- 
ing the  eyes,  is  reading  on  railway  trains.  Here  the  con- 
stant oscillation  of  the  car  causes  an  over  activity  of  the 
muscles,  which  soon  become  exhausted.  Both  the  ex- 
trinsic and  intrinsic  muscles  of  the  eye  are  forced  into  un- 
natural activity'  and  tension.  The  result  ma}'  easily  be 
foretold  ;  the  eves  Avill  sooner  or  later  rebel  and  j^ive  out. 


Individuals  suffering  or  convalescing  from  a  depressing 
illness  or  some  disease  of  the  eye,  should  employ  the  sight 
sparingly.  It  is  not  well  to  subject  a  delicate  ciliary 
mu.scle  to  bear  the  continuous  strain  of  accommodation, 
until  the  other  muscles  of  the  body  have  regained  their 
full  strength  and  firmness.  One  w^ould  not  think  of  taking 
a  long  walk  or  doing  any  hard  manual  labor  when  just  out 
of  a  sick  chamber.  How  much  more  should  the  sensitive 
organs  of  vision  be  guarded  ! 

When  the  ey^s  are  painful  and  there  is  lacrymation, 
or  when  letters  seem  to  run  together,  the  work  should  l)e 
laid  aside  and  the  eyes  directed  to  the  distance.  When 
possible,  outdoor  exercise  and  sleep  should  be  taken. 
Work  should  only  be  resumed  after  a  rest,  and  in  case  the 
symptoms  recur  work  should  be  stopped. 

Distant  vision  is  a  passive  sensation  and  represents  rest, 
and  if  the  sight  is  normal,  is  not  more  exhausting  than 
breathing,  whereas  near  vision  demands  convergence  and 
accommodation,  and  is,  therefore,  a  muscular  effort  and 
requires  exertion.  Care  should  be  taken  not  to  unneces- 
sarily increase  the  strain  by  holding  the  object  too  near 
the  eyes. 

All  the  organs  of  the  bod}^  are  better  for  moderate  and 
judicious  use,  the  eyes  being  no  exception  to  the  rule. 
Normally  constructed  healthy  eyes  should  perform  their 
work  without  the  consciousness  of  the  owner.  The  true 
test  of  this  condition  is  that  there  shall  be  nothing  to  re- 
mind the  user  that  he  has  e3^es. 

No  specific  rule  can  be  laid  dov/n  how  long  one  should 
use  his  e5^es,  as  even  in  health  there  is  such  a  wide  range 
of  individual  difference  in  vigor  and  endurance,  and  what 
may  be  safe  or  moderate  work  for  one  person  would  be  a 
dangerous  excess  for  another.  The  great  peril  of  persons 
who  have  perfect  and  strong  eyes,  just  as  those  who  are 

45 


blessed  with  good  health,  is  overconfidaicc :  such  indi- 
viduals are  very  apt  to  be  imprudent. 

To  preserve  good  sight,  it  is  essential  to  maintain  the 
whole  economy  in  the  highest  state  of  health,  and  the  first 
step  in  this  direction  is  to  have  plenty  of  light  and  fresh 
air,  combined  with  outdoor  exercise  GDod,  nutritious 
diet,  with  sufficient  sleep,  and  proper  division  between 
labor  and  rest,  should  always  be  demanded. 

Late  hours,  dissipation,  fatigue,  and  overcrowded  and 
ill-ventilated  rooms  should  be  avoided.  Those  who  have 
a  predisposition  to  catarrhal  or  rheumatic  ailments  should 
keep  away  from  sudden  changes  of  any  kind,  as  such  eyes 
frequently  become  the  culminating  point  of  disease. 
Violent  affections  or  great  passion,  long-continued  grief 
and  care,  cause  a  diminution  of  eyesight.  These  perpet- 
ualh'  undermine  both  health  and  sight.  Medical  advice 
should  be  sought  in  regard  to  any  disturbance  that  may  be 
traceable  to  weak  eyes.  Headache  and  neuralgia  often 
proceed  from  a  latent  defect  of  eyesight,  or  of  the  eye 
muscles,  although  such  a  sufferer  apparently  has  perfect 
vision  for  both  far  and  near.  Suitable  glasses  will  ofttimes 
give  immediate  relief. 

Eyestrain  may  be  said  to  exist  whenever  errors  of  refrac- 
tion or  mal-adjustment  of  the  ocular  muscles  can  be  de- 
monstrated. It  is  a  frecpient  cause,  and  perhaps,  the  most 
important  of  all  factors,  that  tend  to  produce  functional 
nervous  disease. 


46 


LIGHT. 

Light  is  a  form  of  ethereal  vibration  or  urxdulations  pro- 
duced by  a  luminous  body,  and  propagated  in  all  direc- 
tions with  great  velocit}'.  A  comparative  example  may 
assist  in  comprehending  this  theory  more  clearly  :  If  a 
stone  is  thrown  into  a  smooth  sheet  of  water,  a  series  of 
circular  undulations,  starting  from  the  centre  and  gradu- 
ally enlarging  themselves,  will  be  the  result.  So  it  is  in 
the  case  of  light.  Wlicn  a  luminous  body  is  placed  in 
space  the  ether  which  surrounds  it  is  thrown  into  a  state 
of  vibration,  and  the  motion  is  immsdiately  propagated 
in  all  directions.  It  is  these  undulations  that  excite  the 
retina  and  produce  the  sensations  of  light.  Light,  like 
sound,  is  motion,  while  darkness,  like  silence,  is  rest. 
The  natural  and  greatest  source  from  which  terrestrial  light 
is  derived  is  the  sun,  which  body  is  in  a  constant  state  of 
incandescence. 

All  artificial  sources  depend  upon  the  development  of 
light  during  incandescence.  Every  form  of  matter,  when 
sufficiently  heated,  has  the  power  of  emitting  rays  of  light, 
and  thus  becoming  incandescent  and  self-luminous. 

Light  obeys  a  simple  but  rigid  law.  It  travels,  when 
uninterrupted,  in  straight  lines  at  the  rate  of  185,000  miles 
per  second.  Even  at  this  tremendous  velocit}^,  it  requires 
over  eight  minutes  for  a  flash  of  light  to  reach  the  earth 
from  the  sun. 

When  rays  of  light  emanate  from  an  object  situated  at 
a  distance  of  twenty  feet  (six  metres)  or  more  from  the 
observer,  they  are  considered  as  proceeding  in  parallel  lines, 

47 


and  they  are  supposed  to  enter  the  eye  as  such.  Rays  of 
light  coming  from  an  object  which  is  nearer  than  twenty 
feet  to  the  observer  are  regarded  as  proceeding  from  that 
point  in  lines  which  diverge,  therefore  fall  upon  the  e^e  as 
divergent  rays  of  light ;  the  nearer  the  object  the  greater 
being  the  divergence. 

REFRACTION    OF  LIGHT. 

By  refraction  of  light  is  understood  the  change  of  direc- 
tion a  ray  or  pencil  of  light  undergoes  when  passing  from 

REFRACTION  AND  RPIFLKCTIUN  OF  LKiHT. 


Figure  28. 

Refraction.— (Left-hand  figure).  When  a  ray  of  light  strikes  a  transparent 
object  vertically,  it  passes  directly  through  the  same  without  being  deviated 
in  the  least.  The  incident  ray,  I,  meeting  with  a  pkUe  of  glass  is  bent  toward 
the  vertical,  and  the  refracted  ra>-,  R,  passing  from  the  glass  is  bent  in  tlie 
opposite,  or  from  the  veilical. 

Reflection.— (Riglu-hand  figure).  Should  a  ray  of  light  .strike  a  mirror  or 
polished  surface,  vertically  (in  the  direction  of  arrow),  the  same  would  be  re- 
flected back  on  the  same  path.  If,  however,  the  same  ray  reaches  the  surface 
at  an  angle  of  45  degrees,  it  will  be  reflected  in  the  opposite  direction,  but 
jirecisely  at  the  same  angle. 

one  medium  into  another.  Not  all  transparent  media  re- 
fract light  equalh- ;  when  a  luminous  ray  passes  from  a 
rarer  to  a  den.ser  medium  the  light  is  bent  or  refracted  to- 
ward the  normal  at  the  point  of  incidence ;  whilst  if  it 
pa.sses  from  a  denser  to  a  rarer  medium  it  will  l)e  refracted 
away  from  the  normal  at  the  point  of  incidence.  In  other 
words,  when  an  incident  ray  strikes  a  smooth,  transparent 

48 


surface  vertically  the  beam  passes  straight  through  the 
same,  or  is  unrefracted.  Should  the  beam,  however, 
strike  the  surface  at  an  angle,  the  same  will  be  refracted 
either  toward  or  from  the  vertical,  the  deviation  depending 
upon  the  course  of  the  incident  ray,  as  well  as  the  refrac- 
tive index  of  the  substance.  The  greater  the  difference 
between  the  two  the  more  the  deflection.  It  is  for  this 
reason  that  a  stick  partly  immersed  in  water  appears  to  be 
broken  or  bent  at  the  point  of  immersion.  In  the  study 
of  this  branch  of  .science  the  refraction  of  air  and  crown 
glass  will  alone  be  considered.  If  the  index  of  the  former 
is  taken  as  the  unit,  or  loo,  the  latter  equals  about  150 
(or  50  per  cent.  more).     See  figure  28. 

THE  REFLECTION  OF  LIGHT. 

The  reflection  of  light  is  that  property  by  which  a  ray 
of  light  rebounds  or  is  sent  out  again  when  it  strikes  an 
object.  The  angle  of  reflection  is  always  equal  to  the 
angle  of  incidence.  It  may  be  likened  to  the  action  of  a 
billiard  ball  when  it  strikes  a  cushion — always  glancing 
off  at  an  equal  angle  to  the  original  course.     See  figure  28. 


49 


SPfiCTACLE  LENSHS. 

All  ordinary  lens  or  prism  is  composed  of  a  piece  of 
glass  or  other  transparent  substance,  so  formed  as  to 
change  the  direction  of  rays  of  light  whilst  passing 
through  the  same.  I^enses  are  either  convex  or  concave. 
The  convex  are  designated   as  plus  and  the  concave  as 

FORMS   OR  VARIETIES   OP'  LENSES 


Figure  29. 

A.  Bi-couvex  or  double  convex.  B.  Plano-convex.  C.  Concavo-convex. 

I>.   Bi-concavc  or  double  concave.  E.  Plano-concave. 

1'".  Convexo-Concave.  G.  Piano-prism. 


minus.  The  prefix  sign  !  is  used  to  designate  the  former 
and  the  prefix  sign  —  the  latter. 

Rays  of  light,  when  passing  through  a  lens  or  through 
a  prism,  have  their  direction  altered  ;  tlie}^  are  then  said 
to  have  been  refracted  by  the  lens  or  by  the  prism.  The 
deflection  is  always  toward  the  thickest  part  of  the  lens. 

There  are  two  systems  for  numbering  lenses ;  one  is 
called  the  inch  system  and  the  other  the  dioptric  system. 
The  diopter  is  based  on  the  metric  scale,  and  is  much  the 

50 


better  and  the  more  preferable,  as  it  is  more  scientific. 
At  the  present  time  it  is  used  ahnost  excUisively  by  physi- 
cians. Parallel  rays  from  a  distant  object  passing  through 
a  convex  lens  are  brought  to  a  point  at  F  (see  figute  30), 
and  are  said  to  be  focussed. 

CONVEX  LEXS. 


Fig;nre  30. 
Parallel  rays  are  brniii^ht  to  a  focus  at  F,  on  opposite  side  of  lens. 

This  point  is  therefore  called  the  principal  focus  of  the 
lens  L.  The  space  intervening  between  the  lens  and  its 
principal  focus,  F,  will  depend  upon  the  degree  of  curva- 

CONCAVE  LENS. 


Figure  31. 
Parallel  rays  are  brought  to  a  virtual  focus  at  F,  011  same  side  of  lens. 

ture  of  the  lens.  If  the  distance  between  ly  and  F  is  one 
metre,  the  lens  has  a  focus  of  one  metre,  or  as  commonh^ 
called,  it  has  the  strength  of  one  diopter.  Such  a  lens  is 
employed  as  a  unit  for  calculation. 

When  luminous  rays  pass  through  a  concave  lens  they 

51 


are  refracted  toward  the  thickest  part  of  the  lens,  hence 
parallel  rays  from  a  distant  object  passing  through  such  a 
lens  will  be  refracted  toward  the  periphery  and  pass  as 
divergent  rays.  The  focus,  F,  is  situated  on  the  same  side 
of  the  lens  on  which  the  object  lies  ;  F  would  be  at  that 
point  at  which  the  diverging  rays  cross  one  another  if  they 
were  continued  backward.  This  is  called  the  virtual 
focus.  Hence  the  more  divergent  the  rays  are  after  their 
passage  through  a  concave  lens  the  nearer  will  F  be  to  ly 
(see figure  31).  Consequently  the  more  powerful  the  lens. 
A  lens  of  weaker  power  would  cause  the  rays  to  diverge 
to  a  lesser  extent,  and  if  continued  backw^ard  they  would 

PLANU-FRISM. 


Fijfure  32. 

Prismatic  lenses  displace  objects  ill  llie  (lireciioii   of  the  apex,  or  edge.     The  light 

L,  which  is  situated  directly  on  a  level  with  the  eye,  appears  to  come 

from  the  direction  of  the  dotted  line. 


meet  at  a  point  situated  more  remote  from  the  lens.  Con- 
cave lenses  are  numbered  and  governed  by  the  same  law 
as  convex  ones. 

Should  the  distance  l^etween  L  and  F  be  only  one-half 
metre,  the  lens  would  be  double  the  strength  of  a  i.  I) 
lens,  as  it  is  evident  that  the  lens  that  can  bring  parallel 
rays  to  a  focus  at  a  point  of  a  half  metre  must  necessarily 
have  just  twice  the  refracting  power  of  one  which  focusses 
similar  rays  at  a  point  one  metre  off. 


A  lens  of  one  diopter  ( i.  D )  will  focus  parallel  rays  at 
two        "        (2,  D) 


I  metre, 


three 
four 


(3.D) 
(4.  D) 


etc. 


Therefore  the  higher  the  number  of  the  lens  in  diopters 
the  greater  is  its  refracting  power,  and  consequently  the 
2.  D,  3.  and  4.  D.  have  respectively  two,  three  and  four 
times  the  strength  of  a  one  diopter  lens.  This  law  holds 
good  in  all  forms  of  lenses. 

PLAXO  CVLIXDRirAI.   Li:XSES. 


Figure  ^z- 
A.  Convex  cylitidiical  lens.         B.  Concave  cylindrical  lens. 

These  lei\ses  are  used  for  the  correction  of  astigmatism,  and  ditler  from  spherical 

ietises  because  the  rays  refracted  through  them  do  not  focus  at  a  poiiit, 

but  focus  as  a  line  of  light,  as  shown  in  figure.     (Compare  with 

figure  22.) 


In  the  old,  or  inch  system,  a  one-inch  lens  was  used  as 
the  unit.  Hence  a  No.  10  lens,  or  more  properly  speak- 
ing, a  I -10  inch  lens,  would  focus  parallel  rays  at  10 
inches  ;  a  1-24  inch  lens  at  24  inches,  etc.,  but  as  the  inch 
varied  in  different  countries,  there  was  alwa3^s  consider- 
able confusion,  and  since  the  scientific  world  has  adopted 
the  metric  system 
become  obsolete. 


the  old  way  of  measuring  lenses  has 


53 


OPTICAL    GLASS. 

Crown  glass  is  selected  for  the  superior  brilliancj^  which 
it  possesses,  as  the  glass  intended  for  optical  use  must 
necessarily  be  of  exceptional  transparency.  Great  care  is 
exercised  in  selecting  the  raw  materials  employed  in  the 
manufacture  of  optical  glass,  so  that  they  may  be  as  pure 
as  possible. 

Glass  is  a  transparent,  hard,  brittle  substance  formed 
by  the  fusion  of  silica  (sand )  and  alkalies,  a  minute  por- 
tion of  white  arsenic  or  peroxide  of  manganese  being 
added  on  account  of  their  bleaching  properties. 

Crown  and  plate  glass  are  precisely  of  the  same  compo- 
sition, and  differ  only  in  the  manner  in  which  the  sheets 
of  finished  glass  are  produced. 

The  first  step  in  the  process  of  glassmaking  is  to  fuse 
the  ingredients  in  a  pot  or  crucible.  The  metal  being 
brought  to  a  proper  condition  for  manipulation,  the  opera- 
tor dips  an  iron  tube  or  blow-pipe,  six  or  seven  feet  in 
length,  heated  at  one  end  into  a  pot  of  metal.  With  this 
he  takes  up  the  glass,  and  by  turning  it  gently  around 
gathers  about  two  pounds  of  molten  glass  on  the  end  of  it. 
Having  allowed  this  to  cool,  he  again  dips  the  rod  into 
the  pot  and  gathers  an  additional  quantity.  This  is  also 
permitted  to  cool  as  l:)efore.  The  operation  of  dipping  is 
repeated  until  a  sufficient  quantity,  usually  about  ten 
pounds,  is  gathered.  The  rod,  thus  loaded,  is  held  for  a 
few  seconds  in  a  perpendicular  ]wsition  in  order  that  the 
metal  may  distrilnite  itself  e(|ually  on  all  sides,  and  that  it 
may  be  lengthened  out  beyond  the  rod  by  its  own  weight. 

54 


The  workman  then  molds  the  metal  into  a  regular  form 
by  rolling  it  out  on  a  smooth  iron  plate.  He  then  blows 
strongly  through  the  tube,  and  thus  causes  the  red-hot 
mass  of  glass  to  swell  into  a  hollow  pear  or  crown  shape, 
hence  its  name.  Then,  by  dexterously  revolving  the 
whole  mass  very  rapid  1}^  and  constantly  increasing  the 
velocity  until  the  portion  adhering  to  the  tube  suddenly 
separates  and  flies  open,  the  glass  becomes  a  circular 
plane  or  sheet,  four  to  five  feet  in  diameter,  of  equal  thick- 
ness throughout.  The  sheet,  when  fully  expanded  and 
cooled,  is  then  tempered.  To  reduce  its  brittleness  it  is 
then  annealed.  It  is  subsequently  polished  and  cut  into 
blocks  of  various  sizes.  It  is  then  ready  for  the  grinder's 
lathe  to  be  made  into  lenses. 


55 


SPECTACLE  LENS  GRINDING. 

For  this  kind  of  work  a  special  form  of  lathe  somewhat 
similar  to  those  employed  by  a  lapidist  is  required.  It 
consists  of  a  trough  made  either  of  wood  or  of  metal.     It 

LENS  GRINDKR'S  LATHE. 


Figure  34. 

A.  Table  or  stand.      B.  Trough.      C.  Lever.        D.  Steel  bar,  the  point  of  which 

holds  lens  in  position  on  the  grinding  tool.    K.  Revolving  spindle. 

F.    Tool  or  mold  on  which  the  lens  is  ground. 


is  mounted  on  a  stand  or  table.  (See  figure  34.)  A  verti- 
cal .spindle-box  passes  through  the  bottom  of  the  trough, 
and  contains  a  shaft  having  upon  its  upper  end  a  socket 
for  the  reception  of  the  grinding  tool.     To  the  back  of  the 

56 


trough  is  attached  a  movable  lever  or  handle  with  a  uni- 
versal joint,  and  on  that  part  of  the  handle  situated  di- 
rectly over  the  centre  of  the  grinding  tool  there  is  a 
pointed  steel  bar  which  projects  downward.  This  is  in- 
tended to  fit  into  the  depression  in  the  iron  block,  and 

TOOLS  FOR  C.RIXDI.VG  SPHERICAL  LENSES. 


I^    ^^ 


Figure  35. 

A.  Convex  mold  or  ball  for  grinding  concave  spherical  surfaces. 
P..    Concave  mold  or  cup  for  grinding  convex  spherical  surfaces. 

holds  the  glass  firmly  in   position  on  the  grinding  tool. 
At  the  same  lime  it  permits  it  to  revolve  freely. 

In  the  grinding  of  spectacle  lenses  various  stages  are 

TOOLS  FOR  GRINDING  CYLINDRICAL  LENSES. 


Figure  36. 

A.  Convex  tool  or  mold  for  grinding  concave  cylindrical  surfaces. 

B.  Concave  tool  or  niold  for  grinding  convex  cylindrical  surfaces. 

necessary.  The  glass  passes  through  the  following  pro- 
cedure :  A  piece  of  optical  glass  is  selected  of  the  required 
thickness— about  one  and  one  half  to  two  inches  square. 
This  is  cemented,  with  a  preparation  of  pitch  and  resin, 

57 


to  a  small  iron  block  or  holder,  which  has  a  slight  depres- 
sion (a  shallow  pit)  for  receiving  the  point  of  the  steel 
prong  on  the  lever.  The  next  step  is  to  select  a  mold  of 
the  desired  curve. 

For  grinding  spherical  surfaces,  molds  similar  to  figure 
35  are  required,  and  the  grinding  is  done  by  a  rapidly  re- 
volving spindle.  In  grinding  cylindrical  surfaces  the  mold 
is  fixed  and  the  curve  is  attained  by  a  steady  to-and-fro 
motion.  For  these  purposes  tools  similar  to  those  seen  in 
figure  36  are  requisite.  To  grind  a  concave  lens  a  convex 
mold,  or  ball,  must  be  used. 

To  grind  a  convex  one  a  concave  mold  or  cup  is  selected. 
The  strength  of  the  lens  depends  upon  the  curvature,  and 
not  necessarily  upon  the  thickness  of  the  glass.  The 
grinding  is  done  by  means  of  various  grades  of  emery 
powder,  starting  with  a  coarse  grain  and  finishing  with 
the  finest  flour  emery  until  the  desired  curve  or  focus  has 
been  acquired.  For  polishing  the  lens  a  piece  of  broad- 
cloth is  cemented  to  the  surface  of  the  grinding  tool  and 
coated  with  fine  rouge  moistened  with  water.  The  lens  is 
held  down  with  considerable  pressure,  and  if  the  same 
has  been  well  ground  it  quickly  takes  on  a  very  highl}^ 
polished  surface. 

It  is  almost  needless  to  mention  that  both  surfaces  of  the 
lens  must  undergo  the  same  process,  as  only  one  side  can 
be  ground  at  a  time. 

The  lens  is  now  read}^  to  be  utilized  for  spectacles  and 
eye-glasses,  and  can  be  cut  into  any  desired  shape  or  size. 
A  thin,  pliable  metal  pattern  is  laid  upon  the  lens,  and  all 
superfluous  glass  is  deftly  cut  away.  The  edo:es  are  then 
ground  and  smoothed,  and  the  "lens  is  finished." 


5S 


HISTORY  OF  SPECTACLES. 

The  word  spectacles  is  derived  from  the  Latin  ''spectra,'' 
and  is  defined  as  an  optical  instrnment  consisting  of  two 
lenses  set  in  a  frame,  to  be  worn  for  the  correction  of 
ocular  and  muscular  defects  of  the  eye.  These  important 
aids  to  imperfect  vision  have  been  aptly  termed  "  crutches 
for  the  eye. ' '  The  origin  of  spectacles  is  involved  in  ob- 
scurit}'.  In  all  the  descriptions  of  the  art  of  manufacture 
and  the  numerous  uses  of  glass  collected  from  ancient 
history  there  is  not  a  word  referring  to  the  use  of  specta 
cles.  It  is  probable  that  the  ancients  had  some  knowledge 
of  optics,  or  at  least  that  they  understood  the  use  of  mag- 
nifying lenses  in  some  form.  The  Chinese  claim  that  they 
have  used  spectacles  for  the  relief  of  defective  eyesight  for 
centuries.  This  may  be  so,  but  unfortunately,  like  all 
inventions  made  by  them,  they  were,  in  consequence  of 
their  exclusiveness,  useless  to  the  rest  of  mankind.  The 
only  reference  regarding  lenses  to  be  found  is  recorded  in 
the  Chinese  Chronolo.2:y  of  P.  Gaubel,  in  which  he  tells 
us  that  the  Emperor  Chan,  2283  b.  c,  had  recourse  to  an 
optical  instrument  to  observe  the  planets. 

Among  the  interesting  relics  brought  to  light  in  1854  by 
the  excavation  of  the  ruins  of  ancient  Nineveh,  not  the 
least  remarkable  was  the  discovery  in  the  treasure  house 
of  a  rock  cr^'stal  lens  which  was  found  in  company  with 
bronzes  and  other  articles  of  value.  Its  general  shape 
was  that  of  a  plano-convex  lens,  the  plane  side  having 
been  formed  of  one  of  the  original  faces  of  the  six-sided 

59 


crystal ;  the  convex  side  had  not  been  ground  in  a  cup- 
shaped  tool  in  the  manner  in  which  lenses  are  now  formed, 
but  had  evidentl}^  been  shaped  on  a  lapidary's  wheel,  or 
by  some  such  method.  This  is  proof  that  the  property  of 
lenses  was  known  to  the  ancients,  and  that  they  were  em- 
ployed by  them  long  before  the  Christian  era. 

According  to  Seneca  (4  b.  c. ),  the  ancients  filled  globes 
of  glass  with  water  and  used  the  same  as  lenses  ;  but  no 
mention  is  made  of  their  being  used  as  spectacles. 

One  Roman  historian  reports  that  Nero  (a.  d.  68)  had 
very  defective  vision,  and  at  the  gladiatorial  games  made 
use  of  a  large  jewel,  the  precious  stone  of  which  was 
shaped  in  the  form  of  a  lens,  which  enabled  him  to  see  to 
nuich  better  advantage. 

Among  the  writings  of  Alhazan  Abu  Ali,  who  died 
A.  D.  1038,  at  Cairo,  PCgypt,  there  was  a  work  called 
"Thesaurus  Opticae."  It  is  claimed  that  he  introduced 
the  knowledge  of  spectacles  into  Europe  ;  but  this  is  mere 
conjecture. 

The  year  1280  may  be  assumed  as  the  one  in  which 
spectacles  were  invented,  as  no  trace  of  any  such  invention 
can  be  found  prior  to  that  period.  An  old  Latin  document 
of  the  year  1303,  found  at  the  convent  of  St.  Catherine  of 
Pisa,  records  that  Alexandro  de  Spina,  who  died  in  13 13, 
had  a  pair  of  spectacles  made  for  himself  by  one  who  had 
the  secret  of  their  invention,  but  who  refused  to  make 
known  the  true  process  of  their  manufacture.  De  Spina 
was  so  much  pleased  with  the  spectacles  that  he  made  the 
invention  public.  It  was  through  his  efforts  that  the  em- 
ployment of  spectacles  has  become  known. 

"The  Florentine  Illustrated"  states  that  Leopoldo  del 
Migliore  informs  us  that  the  first  inventor  of  spectacles  was 
Salvino  Armato,  who  died  in  13 17.  This  is  confirmed  by 
the  inscription  on  his  tomb,  which  still  exists  at  Florence. 


Underneath  his  bust,  carved  on  a  large  marble  slab,  may 
be  read  the  following  : 

GUI   GRACE 

SALVING    d'ARMATG   DEGLI    ARM  ATI 

DI    FIRENZE 

INVENTGR    DEGLI    GCCHIALI 

DIG    GLI    PERDONIE   A    PECCATA 

ANNO   DMCCCXVII 

(Here  lies  Salviiio  Aniiato  d'Aiinati,  of  Florence,  inventor  of  spectacles. 
May  God  pardon  his  sins.     The  year  1317.) 

Another  authority  states  that  our  first  positive  knowl- 
edge of  spectacles  is  derived  from  the  writings  and  experi- 
ments of  Roger  Bacon.  He  made  many  discoveries  in 
optics  and  physics,  and  was  the  first  to  describe  a  convex 
lens.  In  describing  spectacles,  he  stated  that  they  were 
useful  to  old  men  and  to  those  who  have  weak  eyes,  for 
it  enables  them  to  see  the  smallest  letters  sufficiently 
magnified. 

Caessmaker  has  published  some  particulars  of  the 
early  history  of  spectacles,  which  differ  from  those  gener- 
ally recorded.  They  are  said  to  be  the  result  of  diligent 
researches  among  records  of  the  convents  and  monasteries. 
According  to  this  writer,^E^©ger/Bac6n  had  spent  some 
years  at  the  convent  of  Cordeliers,  at  Lille,  and  during  his 
stay  formed  a  friendship  with  Henri  Goethals,  known 
better  as  Doctor  Solemnel,  and  with  Philip  Mussche. 
To  these  men  he  paid  annual  visits  during  the  vacations 
at  Oxford,  and  availed  himself  of  the  opportunities  of  pro- 
curing from  the  Belgian  glass  manufactories  fine  glass  fit 
for  optical  purposes.  With  this  glass,  polished  by  himself 
he  made  lenses,  and  communicated  to  his  learned  friends 
the  secret  of  spectacles.  In  this  manner  it  became  known 
in   Flanders  and  to  the  Dominican  monks,  to  whom  the 

61 


Itiliaii  writers  attribute  the  invention.  During  the  pon 
tificate  of  Martin  IV,  who  died  in  1286,  a  question  arose 
touching  the  interest  of  certain  monks,  who  confided  their 
defence  to  Henry  Goethals.  He  being  sixty  years  of  age, 
used  the  spectacles,  the  lenses  of  which  had  been  given  to 
him  by  Roger  Bacon.  Arrived  in  Tuscany,  Goethals 
visited  Nicolas  Messo,  prior  of  the  Dominicans,  and 
stayed  with  them  for  some  two  or  three  weeks.  It  was  in 
this  way  that  Alexandro  de  Spina  became  acquainted  with 
the  use  and  the  ma'iufacture  of  spectacles.  In  the  course 
of  these  researches  it  was  also  discovered  that  the  sister  of 
Goethals,  abbess  of  the  Hospital  of  the  Hermitage,  at 
Eeckergem,  l)eing  dead,  the  religieuses  preserved  for  a 
long  time  her  glasses,  which  were  mounted  in  gold. 

In  a  manuscript  written  in  1299  by  Pissazzo,  the  author 
says  :  "  I  find  myself  so  pres.sed  by  age  that  I  can  neither 
read  nor  write  without  those  glas.ses  they  call  spectacles, 
lately  invented,  to  the  great  advantage  of  old  men  when 
their  sight  grows  weak." 

Another  ancient  document  relating  to  spectacles  is  dated 
in  the  year  1303.  It  is  to  be  found  in  the  Grand  Chirurgie 
of  Gui  de  Chaulic,  which  quotes  the  following,  after  having 
prescribed  certain  eye  salves  :  "If  that  does  not  suffice, 
recourse  must  be  had  to  spectacles." 

Mention  is  made  of  spectacles  in  a  sermon  preached  in 
1305  by  a  Florentine  monk  named  Rivalto. 

Friar  Jordan,  who  died  at  Pisa  in  131 1,  says  in  one  of 
his  sermons,  which  was  published  in  1305  (just  when  he 
preached  the  same  is  not  mentioned),  that  "  it  is  not  much 
over  twenty  years  since  the  art  of  making  spectacles  was 
discovered,  and  is,  indeed,  one  of  the  best  and  most 
necessar}^  inventions  in  the  world." 

It  is  also  recorded  that  .several  cavaliers  of  the  court  of 
Guy  de   Dampierre,  Count  of  Flanders,  at  the  end  of  the 

62 


thirteenth  century,  wore  spectacles,  which  in  those  days 
were  always  mounted  in  gold  or  silver  frames,  and  were 
regarded  as  great  treasures,  receiving  special  mention  in 
wills  and  deeds  (see  foot  note),  and  were  carefully  pre- 
served in  cases  of  ebony  and  silver 

An  old  chronicle  of  Nuremberg,  Germany,  of  the  year 
1482,  mentions  that  there  were  several  manufacturers  of 
spectacles  in  that  city. 

Savonarola,  in  1490,  during  a  discourse,  informs  us  that 
as  spectacles  fell  off,  it  was  necessary  to  add  small  bars  or 
hooks  to  the  frames  to  fix  them  securely  and  prevent  them 
from  falling. 

Whether  the  actual  credit  of  the  invention  of  spectacles 
is  due  to  Alexandro  de  vSpina,  to  vSalvino  Armato,  or  to 
Ro^er  Bacon,  is  not  of  much  consequence,  especially  as 
there  seems  to  have  been  no  distinct  rule  as  to  their  appli- 
cation u:itil  about  the  year  1600.  It  was  not  known  until 
that  period  why  certain  individuals  required  convex  and 
others  concave  lenses.  It  was  left  for  Kepler,  who  died  in 
1630,  to  demonstrate  the  manner  in  which  rays  of  light 
are  refracted  through  the  humors  of  the  eye  and  focussed 
upon  the  retina,  thereupon  forming  a  perfect  image.  He 
pointed  out  the  real  cause  of  myopia  and  hypermetropia, 
and  taught  how  concave  lenses  rectified  the  former  and 
convex  lenses  the  latter. 

The  credit  of  making  spectacles  is  also  accorded  to 
John  Ivippershey,  a  lens-grinder,  of  Middleburg,  Holland, 
who,  on  October  2,  1608,  petitioned  the  government  for  a 
patent  on  his  claim  for  the  invention  of  a  telescope. 


Note. — In  the  inventory  of  the  valuables  of  the  Emperor  Charles  V,  made  after 
his  death,  there  are  enumerated  together  collars  and  badges  of  the  Golden  Fleece, 
various  charms,  as  the  bezaor  stone  against  the  plague,  gold  rings  from  Eng- 
land, good  for  the  cramp,  a  morsel  of  the  True  Cross,  and  twenty-seven  pairs 

of  spectacles. 

63 


It  is  related  of  Spinoza,  who  died  in  1677,  that  he  had 
learned  the  art  of  glass-grinding,  so  as  to  make  a  living 
while  writing  his  philosophical  works,  and  that  he  con- 
structed a  pair  of  spectacles  for  Liebnitz,  who  formed 
his  acquaintance  in  Holland. 

Benjamin  Franklin  has  been  credited  with  devising  the 
first  pair  of  double  focus  (or  bi- focal)  spectacles.  In  a 
letter  dated  May  23,  1785,  he  writes  : (^J^y  Mr.  Dolland's 
saying  that  my  double  spectacles  can  only  serve  particular 
eyes,  I  doubt  he  has  not  been  rightl}^  informed  of  their 
construction.^  I  imagine  it  will  be  found  pretty  generally 
true  that  the  same  convexity  of.  glass  through  which  a 
man  sees  clearest  and  best  at  the  distance  proper  for  read- 
ing, is  not  the  best  for  greater  distances.  I  therefore  had 
formerly  two  pairs  of  spectacles,  which  I  shifted  occa- 
sionally, as  in  traveling  I  sometimes  read,  and  often 
wanted  to  regard  the  prospects.  Finding  this  change 
troublesome,  and  not  always  sufficiently  ready,  I  had  the 
glasses  cut  and  half  of  each  kind  associated  in  the  same 
circle.  By  this  means,  as  I  wear  my  spectacles  constantly, 
I  have  only  to  move  my  e3^es  up  or  down,  as  I  want  to  see 
distinctly,  far  or  near,   the  proper  glasses  being  always 

ready.^This  I  find  more  particularly  convenient  since  my 

being  in  France." 

Thomas  Young,  who  died  in  1829,  published  an  essay 
in  1794,  in  which  he  explained  his  own  case  of  astigma- 
tism. 

David  Brewster,  who  died  in  1S68.  was  among  the 
first  to  correct  astigmatism.  He  found  that  his  own  eyes 
had  myopic  astigmatism,  and  at  his  suggestion  len.ses  to 
correct  this  defect  were  ground  by  Hill,  of  Edinburgh, 
and  by  Pritchard,  of  I^ondon. 

George  B.  Air\-  fully  api)reciate(l  the  importance  of 
correcting  astigmatism  in  the  }ear  1825,  when  describing 

64 


his  own  case  he  states  that  the  vision  of  his  left  eye  was 
so  defective  that  he  was  unable  to  read  or  write,  and  the 
appearance  of  a  candle  flame  was  not  circular  as  when 
seen  with  his  right  eye,  but  was  shaped  like  an  ellipse, 
with  its  long  diameter  at  about  35  degrees. 

It  is  recorded  of  Joseph  Fraunhofer,  who  died  in  1826, 
that  he  purchased  a  machine  to  grind  spectacle  lenses. 

The  researches  of  Bonders,  who  died  in  1889,  created  a 
new  epoch  in  ophthalmology .  His  work,  published  in  1864, 
on  Accommodation  and  Refraction  of  the  Eye,  is  regarded 
as  a  masterpiece,  and  is  the  foundation  of  our  present 
knowledge.  He  explained  the  various  forms  and  anom- 
alies of  refraction,  including  astigmatism,  and  the  method 
of  their  correction. 

Until  twenty  five  years  ago  very  little  attention  was  paid 
to  the  proper  fitting  of  frames.  Great  importance  is  now^ 
not  only  attached  but  demanded  for  the  correct  fitting  and 
centering  of  both  frames  and  lenses. 

Cemented  bi-focal  spectacles,  t.  e. ,  with  a  thin,  supple- 
mental lens  cemented  on  the  lower  portion  of  the  distance 
lens,  as  they  are  worn  to-day,  were  first  introduced  in 
France.  Just  who  ground  the  first  pair  in  America  is  in 
doubt. 

They  came  into  general  use  about  1 880-1 885.  When 
£2;-lasses  are  required  for  both  distant  and  near  vision,  much 
can  be  said  in  favor  of  this  form  of  spectacles,  and  they 
are  worth}^  of  recommendation.  When  the  lenses  are 
made  properly  and  the  frames  are  accurately  fitted,  they 
offer  the  best  method  of  overcoming  the  loss  of  accom- 
modation and  the  necessity  of  constantly  changing  glasses. 
They  are  a  great  convenience,  and  certainly  save  the 
wearer  a  great  deal  of  trouble  and  annoyance.  Patients 
soon  learn  to  use  them  with  great  comfort,  although  occa- 
sionally a  patient  who  cannot  become  accustomed  to  this 

65 


form  of  spectacles,  will  be  met  with.  In  the  adjustment  of 
bi-focal  spectacles  the  habits,  business,  disposition,  and 
personal  peculiarities  have  all  to  be  considered. 

Spectacles  were  for  a  long  time  merely  looked  upon  by 
some  as  mere  objects  of  curiosity,  and  were  made  use  of 
as  a  conspicuous  novelty.  In  Spain  they  formed  part  of 
the  costume  of  every  well-bred  person.  This  absurd  use  of 
glasses  was  meant  to  increase  the  gravity  of  the  appear- 
ance, and  consequently  the  veneration  with  which  the 
wearer  of  them  was  supposed  to  be  regarded. 

The  legitimate  use  of  spectacles  spread  very  slowly, 
because  people  had  little  need  for  them.  Only  a  limited 
number  of  men  could  read  or  write.  Books  were  scarce 
and  costly,  being  only  written  by  hand,  as  printing  was 
not  invented  until  the  early  part  of  the  fifteenth  centur3\ 
The  introduction  and  circulation  of  printed  matter  stimu- 
lated the  demand  for  spectacles,  and  then  their  use  rapidly 
increased. 

At  first  the  lenses  were  very  large,  and  the  frames  were 
exceedingly  heavy  and  clumsy.  Until  the  beginning  of 
the  last  century  no  improvements  deserving  notice  either 
in  the  style  or  weight  of  the  frames  were  made. 

Spectacles  of  exquisite  workmanship  are  now  produced, 
the  combined  weight  of  frame  and  lenses  not  exceeding 
five  to  eight  pennyweights.  The  photographs  herewith 
will  give  an  idea  of  development  in  the  manufacture  of 
spectacles.     (See  figure  37.) 

Too  much  care  and  attention  cannot  be  given  to  the 
preservation  of  sight,  and  it  is  not  too  much  to  say  that 
through  the  aid  of  spectacles  the  enjoyment  of  one  of  the 
most  valuable  of  our  senses  may  be  continued  even  in  old 
age.  Deprived  of  their  help,  most  men  at  the  age  of  fifty 
would  be  too  old  for  work.  In  the  evening  of  life  they 
enable  the  mechanic  to  continue  his  labors,  and  the  artist 

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